Photonic irradiation induces nano-structures an EPR study

Size: px

Start display at page:

Download "Photonic irradiation induces nano-structures an EPR study"

Wilfred Griffith
6 years ago
Views:

1 JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 9, No. 8, August 2007, p Photonic irrdition induces nno-structures n EPR study C. FLOREA *, S. GEORGESCU b ESIEE (Ecole Supérieure d'ingénieurs en Electronique et Electrotechnique de Pris) B.P. 99 F93162 NOISY-LE-GRAND cedex Frnce b NILPRP (Ntionl Institute for Lsers, Plsm, nd Rdition Physics) 409 Atomistilor Street / P.O. Box MG-54 RO Romni During the irrdition of fluorite (CF 2) with photons (X) we observe the formtion of nno ordered structures (inclusions). This forming structure is homothetic in reltion to the fluorine ions of the crystl. The prticles of pproximtely 10 nm size gglomerte nd form nnostructures structures with prllelipipedic shpe. The length of these structures vries between 30 nd 3000 nm. The locl chnges of symmetry produced by the formtion of the nno structures cn be inferred from EPR mesurements on Krmers rre erth ions introduced in CF 2. (Received Mrch 15, 2007; ccepted July 10, 2007) Keywords: Nnostructures structures with prllelipipedic shpe, Krmers rre erth ions, F centers, Locl symmetry chnge, Exciton, G ) tensor, Trigonl xes, Tetrgonl spectrum 1. Introduction A very lrge number of ppers were devoted to the study of the chnges induced in the irrdited crystls. Among the most significnt, in our opinion, re the ppers [1-4]. In these ppers it is evidenced the role of the point defects (essentilly, F centers) in the chnges ppering s the result of the irrdition with high-energy X photons (more thn 1 kev). It seems tht the F centers re produced ccording to the following scenrio [5,6]: the incident photons ionize minly the nionic sites. The nion ioniztion produces t the site positive chrge ( hole or n nionic vcncy). The vcncy trps n electron, which compenstes its positive chrge. The electron nd the vcncy constitute hydrogen-type object with chrcteristic energy levels. The resulting selective bsorption is responsible for the color of the crystl (normlly trnsprent). The ssembly nionic vcncy + electron is nmed F center (from the Germn word "Frbe" for color). To simplify, the F center cn be considered s cubic box contining n electron. The eigenvlues of the electronic energy re (if the potentil vnishes inside the h π 2 box nd is infinite outside): ( n ) 2 x + n y + nz, 2m0 where h is the rtionlized Plnck constnt, m 0 is the electron mss nd n x, n y, nz re non-null integer numbers. In the ground stte n x = n y = nz = 1, i. e. the ground stte is not degenerted. The first excited stte is three times degenerted: (2,1,1), (1,2,1), (1,1,2). The irrdition with photons could distort the site, nd tht is equivlent with locl symmetry chnge. This distortion reduces the symmetry of the F center, removing prtilly the degenercy, nd diminishes the energy of the defect. The energy-configurtion digrm [5] shows tht the stte (1,1,2) representing n elongtion or contrction of the lttice round the F center is predisposed to such distortion. This spontneous distortion of the F center (observed, lso, for other defects or crystls) is of the Jhn-Teller type. Due to this effect the energy of the stte is lowered, the energy being shred with two nionic neighbors, resulting n X 2 (Fluorine, in this cse) molecule oriented long <110> direction. This molecule, very electrophile, trps n electron which forms with the hole nother hydrogen-like stte, nmed exciton. This stte is shred by two neighboring nions. The locl distortion (for instnce, contrction) stbilizes this excited stte denoted (1,1,2). The rditive energy of the de-excittion, diminished by the lttice distortion, could tke the vlue zero (if the configurtiondistortion curves intersect ech other). In this cse the energy of this non-rditive trnsition is completely trnsferred to ions. Tking into ccount the symmetry, the most probble event is the ejection of one of the two ions A in the direction of the molecule X 2. This direction is dense nd formed by A ions of the sme mss. At the time of non-rditive de-excittion, n nion returns to its site while the other one, leving the site, send off the nerest neighbor. The origin of this process is the photon impct; successive impcts propgte with low losses. For enough initil energy (5-8 ev), the initil ion drives off its neighbor, this one drives off its neighbor nd so on. Finlly,

2 2304 fter this series of collisions, vcncy A remins in the initil position while n interstitil is present t some tomic distnces where two nions shre the sme site. The energy of the non-rditive trnsition cn be either found s the energy to crete the two defects (vcncy v nd the interstitil i ) or dissipted by the therml gittion during collisions. As conclusion, we must emphsize tht the cretion mechnism of the nno-structures by irrdition is not yet elucidted. In order to understnd better this mechnism, series of EPR experiments on irrdited smples doped with Krmers rre erth ions (Er 3+ nd Sm 3+ ), hs been crried out. 2. Experimentl C. Flore, S. Georgescu presence of n intense EPR signl chrcterized by G ) tensor with trigonl symmetry. This signl is very intense in the irrdited smple but it is present, with lower intensity, in non-irrdited smple EPR mesurements on irrdited smple The non-doped fluorine smple does not give ny EPR signl. After the irrdition with 50 kev X photons (4 hours nd, respectively, 8 hours) the EPR signls with g 2 re clerly observed. These EPR signls (Figs. 1 nd 2) were recorded for fixed frequency (9.4 GHz) nd vrible mgnetic field ( Guss). The CF 2 single crystls, doped with Er 3+ nd Sm 3+ hve been grown in the Solid Stte Quntum Electronics Lbortory - Ntionl Institute for Lsers, Plsm, nd Rdition Physics. Er 3+ nd Sm 3+ substitute C 2+ in the CF 2 lttice. Smrium enters, lso, s Sm 2+, with nonvnishing Vn Vleck susceptibility (χvv). This susceptibility influences the positions nd the intensities of the EPR signl of Sm 3+. Erbium enters only s Er 3+. The CF 2 single crystls were grown in rgon tmosphere. Prior to the growth, the CF 2 ws purified by therml tretments nd zone melting. The doping concentrtions were 1.5 % mol for Sm nd 0.1 % mol for Er. Smrium ws introduced s metl while Erbium ws introduced s Er 2 O 3. The initil concentrtions (in the nonirrdited smples) of Sm 3+ nd Er 3+ were the sme (0.05 t. % reltive to C 2+ ). Therefore, the intensity of Sm 3+ EPR signl hd the sme order of mgnitude s the Er 3+ signl. For crystlline structures of the fluorite type the EPR studies evidenced the role of the symmetry of the lignd field in the process of the energy trnsfer between the electronic energy levels. By irrditing such crystlline structure with X-rys (~50 kev) we observe tht the symmetry chnge of the lignd field induces the nisotropy of the G ) tensor (tensor of spectrl decomposition). In crystlline structure of fluorine type when the RE 3+ (RE = rre erth) ion enters site with cubic symmetry substituting divlent metl ion Me 2+, the EPR spectr of RE 3+ re rther complex. For Sm 3+ it is difficult to interpret the EPR spectr in the frme of the first order of perturbtion due to the smll gp (~1000 cm -1 ) between the first excited stte nd the ground stte 6 H 5/2. In contrst, the components g nd g of the G ) tensor re well identified. In this cse the gp between the first excited stte 4 I 3/2 nd the ground stte 4 I 15/2 is lrge enough (~6000 cm -1 ) nd thus the first order theory of perturbtions is correct. Due to the chrge compenstion, the site symmetry, initilly cubic, chnges into longitudinl tetrgonl nd/or into digonl trigonl symmetry. Nevertheless, some sites remin cubic. The new result reported in this pper is the b Fig. 1. The EPR signl recorded on CF 2 smple irrdited for 4 hours with 50 KeV X-ry photons. b The Poincré s fit evidences the complex structure of the line: - dominnt Lorentzin contribution (~85%) due to the F centers (thick, solid line); - minor Gussin contribution (~15%) due to the conduction electrons (thin, dotted line).

3 Photonic irrdition induces nno-structures n EPR study Smrium-doped smple In CF 2 the smrium ion (divlent or trivlent) substitutes the divlent ction C 2+. Sm 2+ is not prmgnetic but posses Vn Vleck susceptibility. This susceptibility could produce, in certin neighborhood conditions, shift of the EPR signl of Sm 3+ centers Experiments on non-irrdited smples The EPR spectr were recorded t 4 K for fixed frequency (9.46 GHz) with mgnetic field vrying between 7000 nd Guss. Our mesurements show the presence of centers corresponding to two types of symmetry: ones with cubic nd others with lower symmetry (C 4v ). In the lst cse the G ) tensor corresponds to the xil symmetry of the xes [100], [010] or [001]. We obtined g = 0.90±0.01 nd g = 0.77±0.01. The verge vlue is g ~ long the direction <100>. This vlue is higher thn vlue for the isotropic g, corresponding to the cubic symmetry for Γ 7 (g 0.467). This proves the existence of mgnetic exchnge between Sm 3+ nd Sm 2+. The exchnge provokes, lso, Δg shift. The Sm 3+ spectrum contins, for n rbitrry ngle, three lines (Fig. 3). b Fig. 2. The EPR signl recorded on CF 2 smple irrdited for 8 hours with 50 KeV X-ry photons. b The reltive contributions of Lorentzin (F centers - thick, solid line) nd of Gussin (conduction electrons - thin, dotted line) lines chnges from 85 % : 15 % to 55 % : 45 %. Using the Poincré fit method [8] we put in evidence the complexity of the EPR signl: Lorentzin line (due to F centers) nd Gussin line (due to the conduction electrons) re superimposed. The mesurements evidenced the increse of signl of the conduction electrons s one goes long the formtion of the metl nno-structures in the crystlline structure of CF 2. The CF 2 structure is initilly insulting nd without ny mgnetiztion. Once irrdited, the presence of EPR signl typicl for F centers [1] supports our hypothesis concerning the role plyed by this type of defects in the formtion of the nnometer metl inclusions. The experiments on doped smples llowed to refine our models. Fig. 3. EPR signl of Sm 3+ : CF 2 irrdited 8 hours with 50 kev X photons. For vrious orienttions the vlues of g show n ngulr nisotropy. For n xil symmetry of the crystl field type <100> there re, for n ngle θ, three clsses of resontors, ccording to the three possibilities: [100], [010] nd [001]. If θ is the ngle between [100] direction nd the mgnetic field H r 0, the ngle θ tkes the vlues π π θ, θ nd for the resontors x, y, z.

4 2306 C. Flore, S. Georgescu Experiments on irrdited smples The irrdition with 50 kev X-rys leds to the formtion of point defects. These defects fvor supplementry vlence chnge Sm 2+ Sm 3+. This chnge is produced by n extrinsic mechnism due to interstitil de-loction of Sm or by the vicinity of defect in the lignd lttice. The lst possibility seems to be more probble, the formtion of fluorine vcncies being the most probble process t this irrdition level. As result of the irrdition, globl increses of ~ 10% of the EPR signl due to the increse of the number of resonnt centers. On the other hnd, the line positions re shifted, s result of the new numericl presence of the Sm 2+ ions. The lst effect is provoked by the modifiction (vi the number p of the Sm 2+ nerest neighbors) of the Vn Vleck susceptibility. This suggests tht pproximtely 10% Sm 2+ ions becme Sm 3+. [010] nd [001] crystl xes. The proposed nottions for [100], [010] nd [001] crystl xes re: T X, T Y nd T Z. The four trigonl xes correspond to the directions, 111, [ 111 ] [ ] [ ] nd [ ] Experiments on non-irrdited smples The EPR spectr were recorded t 4 K nd 9.46 GHz. The mgnetic field vries round 1000 Guss. The digrm [ g / sin θ] gives the components of the G ) tensor. The results re given in Tble 1. Tble 1. Components of the G ) tensor in non-irrdited Er 3+ : CF 2. Components of the G ) g tensor g Symmetry Cubic Tetrgonl Trigonl The obtined vlues re close to the vlues reported by Rnon nd Low [7] for the tetrgonl spectrum of the type I, i. e. g = 7.78 nd g = 8.54 (Fig. 5), but we found very different intensities, corresponding to the sme ngulr vlues of the mgnetic field reltive to the T Z xis. For the non-irrdited smple we found tht the Fig. 4. Digrm [ θ] 2 ; sin 2 g for Sm 3+ : CF 2 irrdited 8 hours with 50 kev X photons. In this cse, the chrge compenstion plys determinnt role when the symmetry of the lignd field chnges. The nlysis of the EPR signls of Sm 3+, modified by the symmetry chnge of the sites, is very difficult tsk. As we lredy mentioned, the gp between the ground ste 6 H 5/2 nd the first excited stte 6 H 7/2 is rther smll (~1000 cm -1 ). It is esier to interpret these symmetry chnges for the EPR signls of Er 3+ ions: the energy gp seprting the ground stte 4 I 15/2 nd the first excited stte 4 I 13/2 is much lrger (~6000 cm -1 ) Erbium-doped smple The trivlent Er 3+ ions enter the cubic structure of CF 2 by heterovlent substitution of C 2+. The chrge compenstion induces the formtion of defect which modifies the site symmetry. Three types of symmetry re evidenced: cubic, tetrgonl nd / or trigonl. In tetrgonl symmetry there re three equivlent sites long the [100], Fig. 5. Angulr dependence of the components of the G ) tensor in non-irrdited Er 3+ : CF 2 ; θ is the ngle between [100] direction nd the mgnetic field H r 0. intensities of the signls re in the following rtios: T (tetrgonl) : t (trigonl) : c (cubic) = 4 : 2 : 3. No tetrgonl or trigonl signl of the type II (cited in [7]) ws detected in our spectr.

5 Photonic irrdition induces nno-structures n EPR study Experiments on irrdited smples For the irrdited smples the lines hve the ngulr evolution given I Fig. 6. The ngulr digrms were constructed from the EPR spectr whose prmeters were identified using the Poincré s tretment [8]. 3. Conclusion The comprison of Er 3+ EPR spectr in irrdited nd in non-irrdited CF 2 smples led us to the following conclusions: - the lines conserve their ngulr dependence; - the intensity of the cubic lines decreses; - the intensity of the tetrgonl lines increses slightly; - the intensity of the trigonl lines shows significnt increse; - the totl number of resonnt Er 3+ centers is rigorously the sme in both irrdited nd non-irrdited smples. We put in evidence symmetry chnge of the lignds F from cubic towrd C 4V. The tetrgonl chnge is explined s result of chrge compenstion with n interstitil defect creted by n F- ion occupying the center of the neighbor lignds, initilly empty. In the trigonl cse we suggest tht n O 2- ion replces F ion in cube corner of neighboring fluorine ions. Fig. 6. Angulr dependence of the components of the G ) tensor in Er 3+ : CF 2 irrdited 8 hours with 50 kev X photons. In this cse, the intensity rtio is T : t : c = 8 : 5 : 4. We must emphsize tht our crystls were grown in neutrl tmosphere. The significnt increses of the trigonl signl in the irrdited smples re probbly due to the presence of oxygen. We suppose tht the bsence of the trigonl spectr of type II (whose intensity increse is reported in [7]) is relted to compenstion with oxygen plced digonlly in fluorine position. This oxygen cme from the Er 2 O 3 doping. References [1] F. Beuneu, C. Flore, P. Vjd Rdition Effects nd Defects in Solids, 136, 175, (1995). [2] Ch. T. de Montpreville Note Technique SRPM CEA (1985). [3] E. Johnson, L. Chderton Rdition Effects nd Defects in Solids, 79, 183 (1983). [4] A. Hughes, S. Jin Adv. in Phys., 28, 717 (1979). [5] Y. Quéré Points Deffects in Solids, Msson Ed. Pris (1967). [6] Ch. H. de Novion Note Technique SESI CEA (1993). [7] U. Rnon, W. Low Phys. Rev , (1963). [8] C. Flore Note Technique SESI CEA (1994). * Corresponding uthor: c.flore@esiee.fr

DETERMINATION OF MECHANICAL PROPERTIES OF NANOSTRUCTURES WITH COMPLEX CRYSTAL LATTICE USING MOMENT INTERACTION AT MICROSCALE

Determintion RevAdvMterSci of mechnicl 0(009) -7 properties of nnostructures with complex crystl lttice using DETERMINATION OF MECHANICAL PROPERTIES OF NANOSTRUCTURES WITH COMPLEX CRYSTAL LATTICE USING